(a) Field of the Invention
The present invention relates to a purification method and a purification apparatus for off-gas. More specifically, the present invention relates to a purification method and a purification apparatus for off-gas, capable of lowering the concentration of hydrogen chloride and separating high-purity hydrogen from the off-gas, which is discharged after performing a polysilicon deposition process by a chemical vapor deposition reaction.
This application claims priority to Korean Patent Application No. 10-2014-0009101 filed on Jan. 24, 2014, Korean Patent Application No. 10-2014-0009102 filed on Jan. 24, 2014, and Korean Patent Application No. 10-2014-0051668 filed on Apr. 29, 2014 with the Korean Intellectual Property Office, the contents of which are herein incorporated by reference in their entirety.
(b) Description of the Related Art
One of the known methods for producing polysilicon for solar cells is based on the lamination of polysilicon in a chemical vapor deposition (CVD) reactor, which is also known as Siemens process.
Silicon filaments typically used in the Siemens process is exposed to trichlorosilane with carrier gas at a high temperature of more than 1000° C.
Trichlorosilane gas decomposes and deposits silicon on a heated silicon filament phase, growing the heated silicon filament as shown in Reaction Equation 1 below.2HSiCl3→Si+2HCl+SiCl4  [Reaction Equation 1]
After performing the deposition process of polysilicon by the chemical vapor deposition as described above, chlorosilane-based compounds such as dichlorosilane, trichlorosilane or silicon tetrachloride, or hydrogen and hydrogen chloride are discharged as a reaction by-product.
Off-gas (OGR) containing these chlorosilane-based compounds, hydrogen and hydrogen chloride is generally recovered and recycled through four stages of 1) condensation and compression process, 2) absorption and distillation of hydrogen chloride (HCl), 3) adsorption process of hydrogen (H2), 4) separation process of chlorosilane-based compounds.
More specifically, the off-gas discharged from the polysilicon deposition reactor is transferred to a condensation and compression process, cooled and flowed into a knock-out drum. Separation by temperature is performed, a condensed phase stream of chlorosilane-based compounds is transferred to a distillation column of hydrogen chloride (HCl), and a non-condensed phase stream is transferred to a bottom of the absorption column of hydrogen chloride. At this time, the composition of hydrogen (H2) in the non-condensed phase stream is about 90 mol % or more.
The condensed phase stream where hydrogen chloride component is removed from the hydrogen chloride distillation column is mixed while spraying at the top of an absorption column, and the chlorosilane-based compound component and hydrogen chloride in the non-condensed phase streams are absorbed and removed.
Gas stream where most of the chlorosilane-based compound component and hydrogen chloride has been removed is flowed into a column filled with an activated carbon. The residual chlorosilane-based compound component and hydrogen chloride are absorbed, thus recovering high-purity hydrogen.
Hydrogen purification method described above is a pressure swing adsorption (PSA) process, which is adopted for the separation purification of polysilicon off-gas.
FIG. 1 illustrates a purification apparatus for off-gas according to a prior art.
Referring to FIG. 1, the purification apparatus 300 for off-gas according to the prior art includes a knock-out drum 315, an absorption column 325, a first distillation column 345, an adsorption column 355 and a second distillation column 360.
The off-gas 301 discharged from a polysilicon deposition reactor 305 is cooled in a first cooler 310, flowed into a knock-out drum 315 and divided into a non-condensed gas phase stream 302 containing hydrogen in an excessive amount and a condensed liquid phase stream 303 containing a chlorosilane-based compound in an excessive amount. At this time, most of hydrogen chloride contained in off-gas 301 is distributed in a non-condensed phase stream 302.
The non-condensed gas phase stream 302 discharged from the top of the knock-out drum 315 is additionally cooled and pressurized in a second cooler 320 and then injected into an absorption column 325. At this time, most of hydrogen chloride and chlorosilane components contained in the non-condensed phase stream 302 are removed by a chlorosilane-based stream 307 sprayed in a first distillation column 345 which will be described below. On the other hand, hydrogen stream 304 discharged from the top of the absorption column 325 is finally purified and recycled in the adsorption column 355.
The liquid phase stream 303 discharged from the bottom of the knock-out drum 315 is mixed with a stream 306 discharged from the absorption column 325 via a pump 350 and then injected into a first distillation column 345. A gaseous hydrogen chloride is separated and discharged at the top of the first distillation column 345, and a chlorosilane-based stream 307 where hydrogen chloride has been removed is discharged at the bottom. At this time, the process of the first distillation column 345 consumes more than about 40% of energy of the overall purification process, and is operated with a high-energy process where the most energy is consumed. Most of chlorosilane-based streams 307 are again transferred to the absorption column 325 via a pump 335 and a cooler 330, and used for the absorption of hydrogen chloride and chlorosilane in the non-condensed phase stream 302. The remainder is transferred to a second distillation column 360 and separated into di-/trichlorksilane and silicon tetrachloride, and then recycled.
In the conventional purification method as described above, in order to remove hydrogen chloride contained in the non-condensed stream 302, the condensed phase stream 307 where hydrogen chloride component has been removed from the first distillation column 345 is sprayed and supplied to the absorption column 325. For this process, it is necessary to cool at the absorption column 325 and heat at the first distillation column 345. Thus, there is a problem that the use of energy is inefficient. In addition, in order to ensure the purity of the non-condensed phase, the condensed phase stream from the top of the absorption column 325 is excessively recycled, and thus a major cause is an increase in energy costs for the off-gas purification process.